The inspection of an eye has been widely conducted for the purpose of diagnosing and treating lifestyle-related diseases and diseases that are leading causes of blindness in early stages. As an ophthalmic apparatus to be used for the inspection of the eye, there is a scanning laser opthalmoscope (SLO) using a principle of a confocal laser microscope. The scanning laser opthalmoscope is an apparatus configured to perform raster scanning on a fundus of an eye with laser light that is measuring light to obtain a planar image of the fundus based on the intensity of return light of the measuring light, and the image is obtained with high resolution at high speed. Further, in the scanning laser opthalmoscope, the planar image is generated by detecting only light having passed through an aperture portion (pinhole) out of the return light. This allows only return light at a particular depth position to be imaged, and an image having a contrast higher than that of a fundus camera or the like to be acquired. Such an apparatus configured to photograph a planar image is hereinafter referred to as “SLO apparatus”, and the planar image is hereinafter referred to as “SLO image”.
In recent years, in the SLO apparatus, it has become possible to acquire an SLO image of a retina with improved lateral resolution by increasing a beam diameter of measuring light. However, along with the increase in the beam diameter of the measuring light, an S/N ratio and the resolution of an SLO image of a retina decrease due to an aberration of an eye to be inspected when the SLO image is acquired. The decreases in the resolution are handled by measuring an aberration of an eye to be inspected by a wavefront sensor in real time, and by correcting aberrations of measuring light and return light thereof generated in the eye to be inspected by a wavefront correction device. An adaptive optics SLO apparatus including an adaptive optics system such as the wavefront correction device has been developed to enable the acquisition of an SLO image having a high lateral resolution.
The SLO image obtained by the adaptive optics SLO apparatus can be acquired as a moving image. Therefore, for example, in order to observe hemodynamics non-invasively, the SLO image is used for measurement of the moving speed of blood corpuscles in a capillary vessel and the like through extraction of a retinal vessel from each frame. Further, in order to evaluate a relation with a visual function through use of the SLO image, a density distribution and arrangement of photoreceptor cells P are also measured through detection of the photoreceptor cells P. FIG. 6B is an illustration of an example of the SLO image with a high lateral resolution obtained by the adaptive optics SLO apparatus. In the image, the photoreceptor cells P, a low brightness region Q corresponding to the position of the capillary vessel, and a high brightness region W corresponding to the position of a leukocyte can be observed.
In a case of observing the photoreceptor cells P in such an SLO image, a focus position is set to the vicinity of an outer layer of the retina (for example, layer boundary B5 in FIG. 6A), to thereby acquire such an SLO image as illustrated in FIG. 6B. Meanwhile, retinal vessels and branching capillary vessels travel in an inner layer of the retina (from layer boundary B2 to layer boundary B4 in FIG. 6A). When an adaptive optics SLO image is acquired with the focus position set in the inner layer of the retina, for example, a retinal vessel wall can be observed directly.
However, in a confocal image obtained by imaging the inner layer of the retina, a noise signal is strong due to the influence of light reflected from a nerve fiber layer, and hence it is difficult to observe a blood vessel wall and detect a wall boundary in some cases. In view of the foregoing, in recent years, a method involving obtaining scattering light by changing the diameter, shape, and position of a pinhole arranged in front of a photo-receiving unit and observing a nonconfocal image thus obtained has come to be used (Non Patent Literature 1 (NPL 1)). In the nonconfocal image, a focus depth is large, and hence an object having irregularities in a depth direction, such as a blood vessel, can be observed easily. Further, light reflected from the nerve fiber layer is not easily received directly, and hence noise can be reduced.
Meanwhile, a retinal artery is an arteriole having a blood vessel diameter of from about 10 μm to about 100 μm, and a wall of the retinal artery is formed of an intima, a media, and an adventitia. Further, the media is formed of smooth muscle cells, and travels along a circumferential direction of the blood vessel in a coil shape. Against a backdrop of hypertension or the like, when pressure exerted on the wall of the retinal artery increases, a smooth muscle contracts to increase a wall thickness. At this point in time, when blood pressure is lowered through administration of an antihypertensive agent, the shape of the wall of the retinal artery returns to an original shape. However, when the hypertension remains untreated for a long period, the smooth muscle cell that forms the media undergoes necrosis, and fibrous hypertrophy of the media and the adventitia occurs to increase the wall thickness. At this point in time, an organic (irreversible) dysfunction has already occurred in the wall of the retinal artery, which necessitates continuous treatment so as to prevent an arteriole dysfunction from becoming worse.
Hitherto, a technology for acquiring the nonconfocal image of the retinal vessel through use of the adaptive optics SLO apparatus and visualizing the retinal vessel wall cells is disclosed in NPL 1. In addition, a technology for semiautomatically extracting a retinal vessel wall boundary from an image of an adaptive optics fundus camera through use of a variable shape model is disclosed in Non Patent Literature 2 (NPL 2).
The presence or absence and degree of an organic change in the arteriole need to be estimated in the body of a person suffering hypertension, diabetes, or the like. Therefore, it is desired to simply and accurately measure shapes and distributions relating to the walls, membranes, and cells of the retinal artery being an only tissue that can be observed directly among the arterioles of the entire body. However, in an actual case, the wall thickness and membrane thickness of the retinal artery and the distribution of wall cells of the retinal artery are manually measured from the image acquired through use of an SLO apparatus to which an adaptive optics technology is applied. Therefore, the measurement is complicated, and includes a measuring error caused by an operator, which raises a problem of low reproducibility.
In the technology disclosed in NPL 1, the retinal vessel wall, the membrane boundary, and the wall cells are visualized from an AO-SLO image having a nonconfocal image acquisition function based on pinhole control, and the membrane thickness and a cell density are manually measured. However, a technology for automatically measuring the wall thickness and membrane thickness of the retinal vessel and the density of cells that form the wall is not disclosed.
In the technology disclosed in NPL 2, the retinal vessel wall boundary is detected from the image of the adaptive optics fundus camera through the use of the variable shape model, and the wall thickness of the retinal artery is semiautomatically measured. However, a venous wall, or membranes or cells that form an arterial wall and a venous wall cannot be visualized from the image of the adaptive optics fundus camera. That is, a technology for measuring the wall thickness of a vein, the membrane thickness of the artery or the vein, or the distribution of wall cells is not disclosed even in NPL 2.
Accordingly, there is a demand for a technology for automatically measuring the wall thickness, the membrane thickness, and the distribution of the cells that form the wall from the image obtained by visualizing the blood vessel wall of the eye and the membranes and cells that form the blood vessel wall.